Added or replaced meaningful label for eZDebug messages

Many `eZDebug::write*` calls don't have a label, have a fixed one which e.g. only includes a static string naming the class or are wrong due to copy and paste.

This PR adds `__METHOD__` as a label which makes it easier to check out where a message has been triggered.

In some cases, I also altered the message output, for example when `$this` has been given as message. In these cases, I have transformed the non-string value to `var_export( $myNonStringValue )`.

Cheers
:octocat: Jérôme